When gas carburizing of steel treatment objects is carried out, the carburizing treatment time can be shortened by raising the carburizing temperature. However, if the carburizing temperature becomes too high, the treatment object melts down. For this reason, the carburizing temperature that has been conventionally used in practice was less than the eutectic point temperature (in the case of an iron-carbon equilibrium diagram shown in FIG. 1, it is the C point temperature which is 1147° C.) at which the liquid phase is transformed into γ iron and cementite, and usually was not higher than 1050° C.
In order to shorten the carburizing treatment time at the conventional carburizing temperature, the carburizing treatment was carried out till the concentration of carbon in the surface layer of the treatment object became higher than the final target value, and then diffusion treatment which caused the carbon in the surface layer of the treatment object to diffuse was carried out in a high-temperature atmosphere with a carbon concentration less than that of the carburizing atmosphere, thereby decreasing the carbon concentration in the surface layer to the final target value (Japanese Examined Patent Publication No. 1994-45868).
However, when the carburizing temperature is limited to a temperature less than the eutectic point temperature, the diffusion rate of carbon atoms in the treatment object is restricted. Therefore, the carburizing time cannot be shortened significantly. Thus, the problem is that the diffusion treatment requires a long time, thereby reducing productivity.
It is an object of the present invention to provide a method of gas carburizing capable of resolving the aforesaid conventional problems.